A working fluid supplied during machine processing of a workpiece has various actions as described below. The working fluid has lubricating action in which the working fluid permeates between a workpiece and a tool or between a tool and chips generated by processing to reduce friction at the boundary surface, anti-fusion action in which the working fluid prevents generation of built-up edge or fusion of chips to a tool, and cooling action in which the working fluid absorbs friction heat generated between a workpiece and a tool or between a tool and chips or heat generated in shearing of metal, or the working fluid cools a workpiece or a tool. These actions of the working fluid vary according to the type of the working fluid. An oily working fluid using only oil has strong lubricating action and anti-fusion action and is effective in reducing crater wear of tool rake face or flank wear of a tool. A water soluble working fluid using oil diluted in water has strong cooling action and is effective in reduction of tool wear and improvement in quality of a finished surface. In general, the oily working fluid cannot be used in a high processing temperature since there is a risk of spontaneous ignition of the working fluid, and therefore it is used only in processing at low speed where there is no risk of ignition or in processing of a workpiece for which processing temperature is never high. Although the water soluble working fluid is weaker in lubricating action and anti-fusion action than the oily working fluid, there is no risk of spontaneous ignition of the working fluid and is therefore used more widely. Thus, for machining at high speed or for processing of a hard cutting material for which processing temperature is high, a water soluble working fluid is used.
An example of working fluid previously used in metal cutting is disclosed in Patent Literature 1. The invention described in Patent Literature 1 discloses a water soluble oily metal working fluid. It is described in Patent Literature 1 that the water soluble oily metal working fluid contains a base oil, fine boron nitride powder, and an emulsifier, and the boron nitride powder contains primary particles with a mean particle diameter of not greater than 0.5 μm. Also, it is described that an anionic surfactant, non-ionic surfactant, or mixture thereof is used as an emulsifier.
A homogenizer is a device that pulverizes particles and disperses it in a fluid homogeneously. There are two types of homogenizers, i.e., a pressure-type homogenizer in which a pressurized mixture liquid containing particles impinges to an impact ring at high speed to thereby pulverize the particles finely so as to be dispersed in a liquid, and a rotor-stator-type homogenizer in which a liquid mixture containing particles is supplied to a narrow gap between a rotor and a stator and the rotor is rotated at high speed to pulverize the particles and disperse them in the liquid. A conventional example of a pressure-type homogenizer is disclosed in Patent Literature 2. Patent Literature 2 describes a pressure type homogenizer that uses a valve component having high erosion resistance to disperse hard ceramic powder in a liquid.
A working fluid is generally produced outside of the machine tool in advance, and is stored in a tank belonging to the machine tool until usage. When the quantity of the working fluid in the tank decreases, an operator replenishes the working fluid. A machine tool that is run in automatic operation for a long time may be provided with a working fluid supply apparatus that can automatically replenish the working fluid. An example of working fluid supply apparatus as an auxiliary apparatus of a machine tool is disclosed in Patent Literature 3. Patent Literature 3 discloses production of a working fluid by mixing a raw working fluid supplied from a tank with pressurized water supplied from outside in a jet pump of a mixing machine. It is also disclosed that the raw fluid is forced by the swirling blade provided in the inner cylinder of the jet pump into turbulent swirling flow so that homogeneous dispersion can be achieved.